1. Field of the Invention
The present invention relates to a method of attaching a two dimensional array of leads to the bottom of an integrated circuit package.
2. Description of Related Art
Integrated circuits (ICs) are typically housed within a ceramic or plastic package that is mounted onto a printed circuit board (PCB). The packages have a plurality of leads that are soldered to surface pads on the printed circuit board, to electrically couple the ICs to the board. There are two common types of chip packages, quad flat packs (QFP) and land stud arrays (SGA). Quad flat packs have leads extending from the sides of the package. The leads are typically gull winged (S shaped) and extend around the entire perimeter of the package. The gull wing shape provides flexibility in the leads, which compensates for thermal expansion between the package and the printed circuit board.
As IC's become larger, more leads are required for each package. Increasing the pin count on a quad flat pack, requires either an increase in the size of the package, or a decrease in the spacing between the leads. Increasing the size of the package enlarges the footprint on the card and can expand the overall size of the system. Because of manufacturing tolerances, the reduction of lead spacing typically requires the use of thinner leads. Leads of smaller diameter are subject to higher stresses and failure from vibration, fatigue or shock. Additionally, it is also more difficult to solder the smaller leads to the pads of the circuit board.
Stud grid array packages typically have a two dimensional array of leads extending from the bottom of the package. Although SGA packages provide a high pin count, the leads are straight and rigid. Chip packages typically have a different coefficient of thermal expansion than the board. When the assembly is subjected to temperature changes, the package expands at a different rate than the board. This thermal expansion mismatch induces stresses in the leads, which can cause fatigue and failure at the solder joints. Additionally, the leads of a SGA package are mounted perpendicular to the circuit board. Such an assembly requires coplanarity between the board and the leads. If some leads are shorter than others, the short leads will not make contact with the board, leading to the formation of an inferior solder joint. It is therefore desirable to have a chip package that provides the lead flexibility of a quad flat pack and the high pin count of a SGA package.